AH 5 00 Mo d u le Ma n ua l1 2 - 1 0Impedance discontinuity: The signal at the end of the transmission cable suddenly encounterstoo low impedance or even 0 impedance (E.g.: short circuit) or too high impedance (E.g.: brokencircuit) and then it will cause the signal reflection. To eliminate such kind of reflection, the terminalresistor with the same characteristic impedances as that of the cable must be connected at the endof the cable and make the impedance of the cable in continuity. Since it is a two-way transmissionof the signal on the cable, the other end of the communication cable must also connect a same-size terminal resistor. The following Rt is the terminal resistor.Impedance mismatch: Another reason causing the signal reflection is the impedance mismatchbetween data transceiver and transmission cable. The reflection causes the entire network in amess while the communication circuit is in idle. To prevent the kind of case happening, the upperresistor Rd (connecting the DGND) and the lower resistor Ru (connecting the positive voltage)need be complemented as below. When no data transmission from the station (I.e. in idle state),the complement forces the voltage in different state (I.e. the voltage between conductors) toapproach a fixed value.Connect one station to the PROFIBUS network via the PROFIBUS connector and cable as shownin the following figure.Ru RuRt RtRd RdT he last stati on1st st ationStati on 2Data w ireAData w ire B390Ω390Ω220Ω390Ω390Ω220ΩBA5V543267891DGNDBA5V543267891DGNDBA5 4 3 267891Station 3BA5 4 3 26789112.4.2 Network Topology12.4.2.1 Connecting RS-485 repeater to the PROFIBUS DP networkThe master module could connect 125 slaves at most and its node address could be set to 0~125through SYCON.net software. The node address of the slave could be set to 1~125. If the actuallyconnected devices exceeds 32 on the network or the corresponding baud rate is out of the alloweddistance, the RS-485 repeaters need to be added to expand the physical network. The PROFIBUSDP network is divided into several networks using the RS-485 repeaters. Please note that the twoends of every network should be connected with the terminal resistors respectively. For use of theRS-485 repeater, please refer to the user manual of the used repeater.
